Rotary screen printing systems typically comprise a rotatable cylindrical screen (sometimes referred to as a “printing cylinder”) with an ink squeegee mounted therein. The screen is configured and continuously rotated with respect to a moving web so as to repeatedly print an image on a moving web. In conventional rotary screen printing systems, the rotational speed of the screen is synchronized with the web line-speed. Hence, the size of the image and image repeat length (i.e. the distance between common points of two adjacent repeat images) is determined by the useful printing circumference of the printing cylinder. The theoretical limit of the size of the image and image repeat length is the maximum viable circumference of the screen. However, the entire screen surface is not commonly used for printing. Usually, a section of the screen circumference is blank and non printing. This non-printing region is provided to delineate between individual printed images and to facilitate the joining of different pattern segments.
Accordingly, it is not possible for this type of conventional rotary screen printing system and method to print images with a size and repeat length that is larger than the circumference of the screen. For example, a rotary screen printing system having a screen with a circumference of 1m can not print images with a repeat length greater than 1m. Moreover, this rotary printing system and method can not print images with a “wall height” repeat (typically 2.4m or more).
Large repeats (images have a large size and repeat length) can be obtained using so-called flat printing by means of flat stencils. The product manufactured in this manner might comprise, for example, a bed sheet with a design printed on its head end. The mechanical process of manufacture is laborious and the rate of production thereof is limited.
U.S. Pat. No. 3,990,363 describes one particular solution to the problem of restricted repeat lengths. In this case, the squeegee pressure is released after an image has been printed onto a substrate and is only reapplied when the next repeat image is required. The screen maintains its rotational printing speed when the squeegee is disengaged. Due to the release of squeegee pressure, the pressure with which the screen stencil is in contact with the web is considerably decreased, or even reduced to zero. The problem with this arrangement is that it is difficult to prevent ink seepage through the rotating screen when the squeegee is disengaged from the screen. This results in ink transfer to the substrate between repeats with unsatisfactory contamination of non-print areas on the substrate or soiling of areas printed by a previous print station.
The problem of restricted image size has been solved by reducing the rotational speed of the screen with respect to the web line-speed so as to print a stretched or elongated image on the web. This type of printing process is commonly referred to as “slip” printing. Although the image is larger than the printing region of the screen, the image produced by slip printing is considered to be of an inferior quality.
Designers are presenting ever more challenging designs for printing. For example, designs having a large size format, remotely spaced images, random images and/or multiple colours. In many instances it has not been possible to reproduce these designs using a conventional rotary screen printing system due to the image size limitations, repeat length restrictions, ink seepage problems and the number of print stations required. Hence, to date, these challenging print designs are often only produced using digital printing technologies as opposed to rotary printing screen technology. However, digital printing technologies have their own limitations and can for example, only be used on certain substrates and by using a limited range of inks and ink technologies.
One particularly challenging design for printing, for example on wallpaper, is a large almost continuous design presented over the whole wall length with multiple repeated images at relatively large repeat separations. Using a conventional rotary screen printing process to try and achieve this design would require large numbers of print stations to build up the design in stages. In practice this arrangement would be unsuitable because it would be inherently difficult to control for quality, it would expensive and relatively inflexible.
There is therefore a need for new printing methods and devices to address or overcome one or more of the problems discussed above.